Dimerization of extracellular domains of platelet-derived growth factor receptors. A revised model of receptor-ligand interaction.

The alpha- and beta-subunits of the receptor for platelet-derived growth factor (PDGFR) were found to be autophosphorylated in the absence of ligands at high expression levels which suggests a propensity of PDGFRs to dimerize spontaneously. When the extracellular domains (ED) of both receptors were expressed and purified to homogeneity, they could be dimerized specifically by the different PDGF isoforms. PDGF-BB-induced dimerization was dependent on uncleaved loop I sequences present on both chains. Whereas, in solution, the EDs were weak competitors for PDGF binding to cellular PDGFRs, they formed high and low affinity complexes upon immobilization on solid phase. Cross-competition experiments defined two distinct binding sites on PDGFR alpha-ED. PDGF-AB bound only to the low affinity form of immobilized PDGFR beta-ED and could not dimerize PDGFR beta-ED. Cross-linking studies, however, revealed that both chains of PDGF-AB can interact with a PDGFR beta-ED monomer. Cross-linking of PDGF homodimers with EDs also yielded complexes which contained more than two ligand chains. These results led to a revised model of receptor-ligand interaction and indicate that monomeric PDGF should be able to dimerize PDGF receptors.